1. Field of the Invention
The present invention relates to a network bus bridge and a network system using the network bus bridge, and more particularly to a network bus bridge and a network system which are capable of preventing the bus bandwidth use efficiency from being lowered by the transmission delay.
2. Description of the Related Art
A network having the IEEE1394 high-speed serial bus (hereinafter simply called the xe2x80x9cIEEE1394 busxe2x80x9d) has been paid attention as one of the bus type networks. The IEEE1394 bus has a number of characteristic features such as automatic setting of node IDs, hot plug and play, and isochronous mode suitable for transmission of moving-image data, and has been adopted as a bus for transmitting digital image data.
For the usual arbitration used in IEEE1394-1995, a root node issues a transmission permission. In P1394b, the transmission order of nodes is determined by a procedure called the xe2x80x9cBOSS arbitrationxe2x80x9d. For the BOSS arbitration, a node called xe2x80x9cBOSSxe2x80x9d issues the transmission permission. After the node issues the transmission permission, this node is not the BOSS any more, but the node received the transmission permission becomes a new BOSS.
In other words, the BOSS arbitration is one kind of token passing. Not becoming the BOSS by giving the transmission permission to another node corresponds to xe2x80x9ctossing a tokenxe2x80x9d, and becoming the BOSS by receiving the transmission permission corresponds to xe2x80x9creceiving a tokenxe2x80x9d.
A different point of the BOSS arbitration from the general token passing is that in the token passing the circulation order of the token is predetermined, whereas in the BOSS arbitration the node becoming the BOSS (receiving a token) is determined each time through competition. Namely, in the BOSS arbitration, a node which does not desire to transmit data does not become the BOSS (i.e., does not receive a token).
Generally, a half-duplex bus type network has advantages that broadcast is easy and the network topology is flexible, but has disadvantages that other nodes cannot transmit data while one node transmits a frame of a packet. The period while a node transmits a frame is more precisely xe2x80x9ca period from when a frame starts being transmitted to when it becomes that the next frame can be transmittedxe2x80x9d. This period can be generally defined as a period from the time instance when a node starts transmitting a frame to when the transmitted frame extinguishes from a bus. Namely, when the bus is broadened (i.e., the maximum value of a delay between nodes is set large) for long distance transmission, the transmission delay time becomes long. As a result, the xe2x80x9cperiod from when a frame starts being transmitted to when it becomes that the next frame can be transmittedxe2x80x9d is prolonged by the transmission delay. This is apparent from the following equation [1] which indicates the time for the same size frame to be transmitted to the whole bus:
(Frame size)/(Transmission Rate)+Transmission delayxe2x80x83xe2x80x83[1]
It can be understood that when the transmission delay occurs, an additional time equal to the transmission delay requires transmitting the same size frame, so that the occurrence of the transmission delay lowers the bandwidth efficiency. This problem is troublesome in that the higher the transmission rate is, the larger the influence of the transmission delay is, as seen from the equation [1].
In order to cope with this lower efficiency, a large frame is generally used. Because the influence of the transmission delay can be mitigated as the frame size becomes large, as apparent from the equation [1]. The large frame can therefore suppress the transmission efficiency from being lowered.
A network using the IEEE1394 bus is one of the half-duplex bus type networks. This network therefore has the characteristics that if the transmission distance is elongated to generate the transmission delay, the bandwidth efficiency lowers.
In the BOSS arbitration of P1394b, the BOSS can transmit a frame. After the BOSS transmits the transmission permission to another node after transmitting the frame, it is not a BOSS any more. The node received the transmission permission becomes a new BOSS and can transmit a frame immediately thereafter. Namely, in P1394b, a period while a frame cannot be transmitted is a period from the time instant when the BOSS transmits the transmission permission to the time instant when another node receives the transmission permission and becomes the new BOSS. During this period, there is no BOSS on the bus. Therefore, any frame will not be transmitted. This will be described in more detail with reference to FIGS. 13 and 14.
FIG. 13 is a diagram showing an example of a network system using the IEEE1394 bus, and FIG. 14 is a diagram showing an example of the operation timing of the network system. In the network system shown in FIG. 13, four nodes xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d are connected to the IEEE1394 bus. The node xe2x80x9cBxe2x80x9d is a cycle master. In FIG. 14, a both-head arrow in the uppermost area indicates a transmission cycle of 125 xcexcs. During the time duration xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d, xe2x80x9cDxe2x80x9d, xe2x80x9cAxe2x80x9d, xe2x80x9cDxe2x80x9d, xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cAxe2x80x9d indicated by both-head arrows in the second uppermost area, nodes corresponding to these alphabets become the BOSS. Namely, they become the BOSS in the order of node xe2x80x9cBxe2x80x9dxe2x86x92node xe2x80x9cCxe2x80x9dxe2x86x92node xe2x80x9cDxe2x80x9dxe2x86x92node xe2x80x9cAxe2x80x9dxe2x86x92node xe2x80x9cDxe2x80x9dxe2x86x92node xe2x80x9cBxe2x80x9dxe2x86x92node xe2x80x9cCxe2x80x9dxe2x86x92node xe2x80x9cAxe2x80x9d. Idle periods caused by the transmission delay are indicated by both-head arrows (in the third uppermost area) during the period when each of the nodes becomes the BOSS. The transmission timings of nodes xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d are indicated by xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d affixed to the leftmost area in FIG. 14.
The operation of the network system shown in FIG. 13 will be described. After the node xe2x80x9cBxe2x80x9d which is the cycle master and the BOSS transmits a cycle start packet CS on the bus, it transmits a transmission permission token on the bus. The cycle start packet CS and the transmission permission token are flowing on the bus and are received by the nodes xe2x80x9cAxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d. The node xe2x80x9cCxe2x80x9d receives the transmission permission token to become the BOSS. There-after, the node xe2x80x9cCxe2x80x9d transmits an isochronous packet IsC and the transmission permission token on the bus. Similarly, the nodes xe2x80x9cDxe2x80x9d and xe2x80x9cAxe2x80x9d become thereafter the BOSS in this order. The node xe2x80x9cDxe2x80x9d transmits an isochronous packet IsD and the transmission permission token on the bus, and the node xe2x80x9cAxe2x80x9d transmits an isochronous packet IsA and the transmission permission token on the bus. After the nodes xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d, xe2x80x9cDxe2x80x9d and xe2x80x9cAxe2x80x9d sequentially transmit isochronous packets on the bus in the above manner, the nodes xe2x80x9cDxe2x80x9d and xe2x80x9cCxe2x80x9d sequentially transmit asynchronous packets AD and AC. In transmitting the asynchronous packet, it is determined that the node which receives the asynchronous packet returns an acknowledgement packet to the transmitting node. Therefore, for the asynchronous packet AD, the node xe2x80x9cBxe2x80x9d becomes the BOSS and returns the acknowledge packet to the node xe2x80x9cDxe2x80x9d, and for the asynchronous packet AC, the node xe2x80x9cAxe2x80x9d becomes the BOSS and returns the acknowledge packet to the node xe2x80x9cDxe2x80x9d.
It can be understood from the above explanation that the idle time period is a period while the BOSS does not exist. In the case of the token passing, the idle period is a period while a token flows on the network. Therefore, the above-described general formula [1] becomes the following formula [2] for the BOSS arbitration:
(Frame size)/(Transmission rate)+(Transfer time of transmission permission token)xe2x80x83xe2x80x83[2]
In the case of the IEEE1394 bus, a long distance such as a large bus size will not cause any deterioration of the efficiency due to the long distance transmission delay if the nodes which become the BOSS (which desire to transmit) are concentrated in a narrow area. The reason is as follows. The idle time is a time for which the transmission permission token is transferred from the current BOSS to the next BOSS. Therefore, only the transmission delay between the nodes which become the BOSS (which desire to transmit) becomes an issue, and the bus size is not directly related to the efficiency. Namely, the following inequality [3] is established:
(Value of equation [1])xe2x89xa7(Value of equation [2])xe2x80x83xe2x80x83[3]
However, it is generally rare that the nodes which become the BOSS (which desire to transmit) are concentrated in a narrow area of a broad bus. It is usual to consider that the nodes exist in an area from one end to the other end of the bus. The influence of the bus size is therefore hard to be reduced, so that it is not substantial that the value of equation [2] is much smaller than that of equation [1]. It can be said that the BOSS arbitration is much more efficient than the general token passing.
One approach to solving this problem of the low efficiency is to make large the frame size similar to the general network system. The larger the frame size is, the less the influence of the transfer time of the transmission permission token is.
However, the IEEE1394 bus is associated with a critical problem. As shown in FIG. 14, the IEEE1394 bus has the cycle of 125 xcexcsec. Data transfer is executed by the repetitive transmission of a small frame in each cycle in order to suppress the generation of the jitter. For example, if data is to be transmitted at the bandwidth of 6 Mbps, it is necessary to transmit a frame having at least a size of 6 Mbpsxc3x97125 xcexcs=750 bits=94 bytes (4 bytes+header size) in one cycle. This cycle poses the following limit of the IEEE1394 bus expressed by the following inequality:
(Total of values of equation [2] for all frames to be transmitted in one cycle)xe2x89xa6125 xcexcsxe2x80x83xe2x80x83[4]
A more specific representation of inequality [4] becomes the following equation:
(Number of packets to be transmitted in one cycle)=k93 [(Frame size of packet k)/(Transmission rate)+(Transfer time of transmission permission token from BOSS(k) to BOSS(k+1))xe2x89xa6125 xcexcsxe2x80x83xe2x80x83[5]
Namely, if the frame size is to be made large, it is necessary not only to satisfy the limit of a frame size defined by the specification but also to satisfy the condition defined by inequality [5]. It means that as the number of frames to be transmitted in one cycle increases too large, the frame size cannot be-made large. Since the IEEE1394 bus has this problem of an inability to make the frame size large, another solution method is required.
The present invention has been made in order to solve the above problems. An object of the present invention is to provide a network bus bridge and a network system which are capable of preventing the bus bandwidth use efficiency from being lowered by the transmission delay in the half-duplex bus type network system such as the IEEE1394 bus.
The first network system of the present invention comprises: one transmission node; one or more reception nodes; and a half-duplex bus connected between the transmission node and the reception nodes. With this configuration, the one-way transmission is performed from the transmission node to the reception node, so that the bus bandwidth use efficiency can be prevented from being lowered by the transmission delay and the transmission efficiency can be improved.
The first network bus bridge of the present invention comprises one or more transmission portals. With this configuration, the one-way transmission from the transmission portal to a half-duplex bus becomes possible.
The second network bus bridge of the present invention comprises one or more reception portals. With this configuration, the one-way transmission from a half-duplex bus to the reception portal becomes possible.
The second network system of the present invention comprises: a first network bus bridge having one transmission portal and one transmission/reception portal; a plurality of second network bus bridges each having one reception portal and one transmission/reception portal; and a half-duplex bus connected between the transmission portal and the reception portal. With this configuration, by realizing the one-way transmission between the transmission portal of the first network bus bridge and the reception portal of the second network bus bridge, the transmission efficiency can be improved.
The third network system of the present invention comprises:
a first network bus bridge having one transmission portal and one transmission/reception portal;
a second network bus bridge having one reception portal and one transmission/reception portal;
a third network bus bridge having one transmission portal and one transmission/reception portal; and
a fourth network bus bridge having one reception portal and one transmission/reception portal, wherein
the transmission portal of the first network bus bridge and the reception portal of the second network bus bridge is connected via a half-duplex bus;
the transmission portal of the third network bus bridge and the reception portal of the fourth network bus bridge is connected via a half-duplex bus;
the transmission/reception portal of the first network bus bridge and the transmission/reception portal of the fourth network bus bridge is connected via a half-duplex bus; and
the transmission/reception portal of the second network bus bridge and the transmission/reception portal of the third network bus bridge is connected via a half-duplex bus.
With this configuration, by realizing the one-way transmission from the transmission portal to the reception portal, the transmission efficiency can be improved. Also, by providing a pair of the one-way transmissions in the reverse direction, the two-way transmission with the high transmission efficiency can be realized.
The third network bus bridge of the present invention comprises: one or more transmission portals; and one or more reception portals. With this configuration, the one-way transmission from the transmission portal to a half-duplex bus and the one-way transmission from the reception portal to the half-duplex bus become possible.
The fourth network system of the present invention comprises two third network bus bridges, wherein the transmission and reception portals of one of the two third network bus bridges are connected to the reception and transmission portals of the other via half-duplex buses, respectively; and the transmission/reception portals of the two third network bus bridges are connected via a half-duplex bus. With this configuration, by realizing the one-way transmission from the transmission portal to the reception portal, the transmission efficiency can be improved. Also, by providing a pair of the one-way transmissions in the reverse direction, the two-way transmission with the high transmission efficiency can be realized.
The fourth network system of the present invention comprises three or more third network bus bridges, wherein the connections of the transmission and reception portals of one of two adjacent network bus bridges to the reception and transmission portals of the other of the two adjacent network bus bridges via half-duplex buses are repeated to form a loop along which data flows in one direction. With this configuration, by realizing the one-way transmission from the transmission portal to the reception portal, the transmission efficiency can be improved and the loop type transmission can be realized.